Whole shale oil contains distillate and residuum fractions with widely different compositions beginning with light hydrocarbons typically boiling in the C4 range and extending to a wide range of higher distillate boiling hydrocarbons and heteroatomic compounds up to and including compounds boiling in the 975° F.+ vacuum residuum range. These wide boiling ranges of hydrocarbons and heteroatomic compounds can have widely varying reactivities in downstream catalytic or thermal upgrading processes. Raw shale oil may contain nitrogen containing compounds, metals such as arsenic and/or selenium and compounds of arsenic and/or selenium and other impurities such as sulfur containing compounds. Additionally, raw shale oil may also contain particulate matter comprised of fine oil shale particles that are entrained or trapped in the recovered whole shale oil products from the upstream retorting or in situ extraction processes. The hydrocarbons in whole shale oil may include various paraffins, olefins, diolefins, and aromatics, including heavy oil, gas oil, and asphaltenes containing multiple fused aromatic ring compounds.
Whole shale oils are conventionally processed in a single hydrotreating reactor. The reactor is operated at a single hydroprocessing severity, which, however, cannot effectively perform the required hydroprocessing reactions to upgrade the whole shale oil without encountering the processing penalties of severe fouling/plugging and poor selectivity, e.g., gas formation. This is a result of the different compositions and reactivities of the various fractions in the whole shale oil.
Other processes disclose use of multiple reactors to upgrade the whole shale oil. For example, U.S. Pat. No. 4,133,745 discloses a process for processing shale oil in which the shale oil is fractionated into a naphtha cut, boiling below 350° F., and a gas oil cut, boiling above 350° F. The naphtha cut is then hydrotreated to remove nitrogen and the gas oil cut is treated to remove impurities, such as by caustic treating. The gas oil cut is then hydrotreated to remove nitrogen compounds and fractionated to produce a second naphtha cut boiling below 450° F. While this approach manages to hydroprocess all but a minor portion of the light naphtha portion, it suffers from the attendant deficiencies of fouling, plugging, and selectivity issues.